The British Rail Class 73 is a British model of electro-diesel locomotive. The type is unusual in that it can operate from the Southern Region's 650/750 V DC third-rail or an on-board diesel engine to allow it to operate on non-electrified routes.

This class of 49 locomotives was built in two batches using English Electric components. The first six locomotives were built at Eastleigh works in 1962. Following successful trials of the initial locomotives, a production run of 43 was built by English Electric at their Vulcan Foundry between 1965 and 1967. They differed slightly from the six earlier machines, most notably having an increased tractive effort as well as a higher maximum speed (90 mph as opposed to 80).

The Class 73’s were able to close couple with Southern Region electro-pneumatically controlled electric multiple units and diesel electric multiple units for push-pull train operation - the reason for retractable buffers.

The narrow box-like body allowed use all over the Southern Region network including through the narrow tunnels on the Hastings Line.

Two locomotives were completely rebuilt by Rail Vehicle Engineering Limited (now part of Loram UK) of Derby. The existing diesel engine was removed and replaced by a pair of Cummins diesel engines, increasing the total diesel horsepower to 1,500.